1. Field of the Invention
The present invention relates to a method and apparatus for polishing substrates. More particularly, the invention relates to a buffer station associated with a metrology device on a chemical mechanical polishing tool to improve throughput of the chemical mechanical polishing tool.
2. Background of the Related Art
In the fabrication of integrated circuits and other electronic devices, multiple layers of conducting, semiconducting and dielectric materials are deposited and removed from a substrate during the fabrication process. Often it is necessary to polish a surface of a substrate to remove high topography, surface defects, scratches or embedded particles. The polishing process is often referred to as chemical mechanical polishing (CMP) and is used to improve the quality and reliability of the electronic devices formed on the substrate.
In general, the polishing process involves holding a substrate against a polishing pad under controlled pressure and temperature in the presence of a slurry or other fluid medium. Typically, the polishing process involves the introduction of a chemical slurry during the polishing process to facilitate higher removal rates and selectivity between films on the substrate surface. One polishing system that is used to perform CMP is the Mirra.RTM. CMP System available from Applied Materials, Inc. and shown and described in U.S. Pat. No. 5,738,574, entitled "Continuous Processing System for Chemical Mechanical Polishing", the entirety of which is incorporated herein by reference.
One problem encountered in polishing processes is the non-uniform removal of films from the surface of substrates. Removal rate is related to downward pressure on the substrate, rotational speeds of the polishing pad, slurry composition including particle density and size and the effective area of contact between the substrate surface and the polishing pad. Removal rates also tend to be higher at the edge of the substrate as opposed to the center of the substrate.
Another problem encountered in CMP is the difficulty in removing non-uniform layers which have been deposited on the substrate. During fabrication of electronic devices on a substrate, layers are deposited or grown in certain regions on a substrate which may result in an uneven topography over the surface of the substrate. A CMP process can be employed to remove the unwanted topography resulting in a substantially planar surface on the substrate. The thickness of the films deposited on the substrate can be very thin (on the order of 0.5 to 5.0 microns), thereby allowing little tolerance for non-uniform removal. A similar problem arises when attempting to polish warped surfaces on a substrate. Substrates can become warped during the fabrication process due to exposure to various thermal cycles. Due to the warpage experienced by the substrate, the substrate will have high and low areas and the higher areas will be polished to a greater extent than the lower areas.
In order to monitor the production of the electronic devices and in particular to monitor the results of polishing processes, pre and post monitoring systems have been included on CMP systems. One such monitoring system is available from Nova Measuring Instruments Ltd. located in Israel (also Nova Measuring Instruments, Inc. located in Sunnyvale, Calif.) and sold as the NovaScan 420. The NovaScan 420 has been used to advantage in a Mirra.RTM. CMP System and provides a wet substrate inspection process to inspect substrates following processing. One disadvantage with available monitoring systems is their single substrate capacity. As a result, the throughput, i.e., the number of substrates processed per unit of time, is lower than desirable. The available monitoring systems have a single substrate support to receive a single substrate for processing. As a result, the robot transfer system must perform numerous movements to effectuate transfer of an evaluated substrate to a storage location within the system and then retrieve and position another substrate in the monitoring system. The additional movements required of the robot transfer system increase the idle time of the monitoring system and increase the amount of time required for substrate handling, thereby decreasing throughput of substrates in the CMP system.
Therefore, there is a need for a CMP tool having a buffer station for supporting one or more substrates to increase the throughput of a CMP tool.